One could argue that adults from the third world are saner/happier than adults from the first world, despite poorer material conditions, because the struggle to obtain food, survive war and so on keeps them more more in contact with reality.

Does this mean we should deliberately starve western adults, or introduce artificial viruses and other stressors into their environment, in order to improve their mental well-being?

Of course not. The reality is that there are many urgent problems of survival for all humans, for example: supervolcanoes, ice ages, asteroid impacts, cancer, genetic meltdown, local supernovae, and an unlimited supply of others yet unknown. The physical universe is hostile to first worlders and third worlders alike, and to adults and children alike. The social and bureaucratic environment can be pretty bleak too!

Therefore we need the material and technological space we inhabit to allow us the best chance of solving these problems. We also need to find ways to make them seem less abtract and remote, so that we engage with them as if our lives and our sanity depended on it. Because they do.